1. Field of the Invention
The present invention relates generally to engine cooling apparatuses and, more particularly, to an expansion tank that absorbs pressure variations occurring with volume change of coolant that circulates through the engine cooling apparatus, and separates the coolant into gas and liquid.
2. Description of Related Art
A construction machine such as a hydraulic excavator generally includes an engine serving as a prime mover, and an engine cooling apparatus that cools the engine by allowing coolant to circulate through a coolant circuit between the engine and a radiator. The coolant circuit often includes a hermetic reservoir tank (what is called an expansion tank) that removes air from the coolant and make an internally reserved air chamber act as an air spring to absorb pressure variations occurring with volume change of the coolant.
The expansion tanks in the conventional technique have a plurality of separate chambers partitioned from each other by a bulkhead. Such separate chambers each have a coolant communication hole in the bottom of the chambers, the coolant communication hole providing coolant circulation between the separate chambers. In addition, the separate chambers each have an air communication hole in an upper portion of the chambers, the air communication hole providing air circulation between air chambers reserved in the respective separate chambers. The coolant introduced into the expansion tank from the coolant circuit, while flowing through the separate chambers, is subjected to gas-liquid separation before being delivered to the coolant circuit.
In such an expansion tank, however, the air chamber reserved in each separate chamber has a capacity varying with an amount of coolant supplied to the expansion tank. Thus, when an excessive amount of coolant is supplied to the expansion tank, the capacity of the air chamber reserved in each separate chamber is reduced. As a result, pressure variations occurring with the volume change of coolant will be insufficiently absorbed, so that an inordinately increased internal pressure of the coolant circuit damages some parts of the coolant circuit. As a solution to this problem, Japanese Patent No. 3867607, for example, discloses an expansion tank that can absorb pressure variations occurring with the volume change of coolant even when an excessive amount of coolant is supplied by a user.
The expansion tank disclosed in Japanese Patent No. 3867607 includes a separate chamber that only has a coolant communication hole. This arrangement of the coolant communication hole in the separate chamber provides an air chamber at the area higher than the hole regardless of the amount of coolant supplied to the expansion tank. The pressure variations occurring with the volume change of coolant can thus be absorbed even when an excessive amount of coolant is supplied to the expansion tank.